run-command: report exec error even on ENOENT

run-command: report exec error even on ENOENT

If execve(2) fails with ENOENT and we report the error, we use the
format "cannot run %s", followed by the actual error message.  For other
errors we use "cannot exec '%s'".

Stop making this subtle distinction and use the second format for all
execve(2) errors.  This simplifies the code and makes the prefix more
precise by indicating the failed operation.  It also allows us to
slightly simplify t1800.16.

On Windows -- which lacks execve(2) -- we already use a single format in
all cases: "cannot spawn %s".

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>